The present invention relates to a screw extruder with a feed bush axially surrounding an extruder screw. The feed bush is arranged in the area of the material intake and comprises several longitudinally extending grooves distributed around its inner circumference in the circumferential direction. The invention also relates to a method for controlling extrusion in a screw extruder.
A screw extruder of this type is described in German Pat. No. 3,226,918. The extruder is designed so that the feed bush, seated in the extruder barrel and axially surrounding the extruder screw, processes various material grades relative to the extruder screw with various twist effect. For this purpose, the feed bush is subdivided into a rotatable longitudinal section and a rotationally fixed longitudinal section. The rotatable longitudinal section is mounted downstream, in the feed direction, of the material intake hole, rotatable relative to the extruder barrel and the extruder screw. The rotationally fixed longitudinal section has one zone upstream and one zone downstream of the material intake hole. The rotational movement of the rotatable longitudinal section of the feed bush is determined either by a rotary drive or by a brake. The feed grooves may have a uniform width over their entire length or else a width dimension converging in the feed direction which is preferably designed such that the increase in cross-sectional width is inversely proportional to the decrease in cross-sectional height. In this case, the cross-sectional height of the feed grooves of the rotationally fixed section and the rotatable longitudinal section decreases continuously over the entire length of the feed grooves, i.e., the end of the feed grooves in the rotationally fixed longitudinal section has the same cross-sectional height as the beginning of the feed grooves in the rotatable longitudinal section. The cross-sectional width of the feed grooves in the rotationally fixed longitudinal section is many times greater at its end than the cross-sectional width of the feed grooves in the rotatable longitudinal section at its beginning. The cross-sectional widths of the feed grooves in the rotatable longitudinal section are many times wider at their end than at their beginning. With this design of the feed grooves, the cross-sectional height is fixed over the entire length of the feed grooves so that an adaptation of the processing speed to various grades of the plastics material to be processed can only be achieved by regulating the rotational movement of the rotatable longitudinal section of the feed bush. This involves twist-like displacement movements between the feed grooves of the rotatable longitudinal section of the feed bush and the circumference of the extruder screw. It is understood, therefore, that a blockage of the groove cross-sections between the fixed longitudinal section and the rotatable longitudinal section must be avoided.
European Published Application No. A2-0,069,271 describes a screw extruder with an extruder barrel in which a rotatable extruder screw is mounted. Grooves which have a triangular cross-section and extend spirally are arranged in the inner circumferential surface of the feed zone or of the feed bush of the extruder barrel, in the feed zone upstream and downstream of the filling hole for the plastics raw material. The grooves extend from a maximum groove depth at the beginning of the feed zone to a groove depth equal to zero at the end of the feed zone. The shape, i.e., the longitudinal cross-section of the grooves and their number, is thus fixed and can only be changed by exchanging the extruder barrel.
As is known, in plasticizing extruders, raw plastic material is conveyed and consolidated in the feed zone and melted in the compression or melt zone. In the metering zone, the melt is homogenized and brought to the necessary die pressure. In shaping of the feed zone, two concepts are customary in the latest state of the art, namely, the smooth screw or extruder barrel used in conventional application, and the grooved barrel, used in positive or forced conveying application.
The grooved extruder barrel was used, as a first application, in the processing of hard to convey plastic powder-primarily high-molecular polyethylene-which could not be processed completely in conventional extruders. Nowadays, the advantages of this method mean that forced conveying is used not only for plastic powder but also for many plastic granules. In general, this method is always suitable if the coefficient of friction between the plastic particles is greater by a certain amount than the coefficient of friction between the plastic fill and the metallic walls of the extruder barrel. If this is the case, a much higher throughput is provided than with the conventional method, and it is substantially independent of the counterpressure, i.e., die pressure. However, as is known, force conveyance only functions if the grooves of the feed zone are intensely cooled so that an initial or even complete melting of the plastic particles is avoided.
According to the state of the art, at an inside diameter D of the feed bush, the grooved feed zone has a length of L=2 to 4 D, the grooves extending either axially parallel or helically. However, there have also already been described feed lengths of up to L=5 to 6 D, which make possible greater screw channel depths and thus, in the final analysis, greater throughputs with pressure independency of the output. As a rule, the grooves made in the feed bush are of conical design, i.e., they taper continuously from the deepest dimension at the channel entrance to the end of the feed zone. Grooves with rectangular, triangular, semi-circular or crescent-shaped profiles are often used. The grooves are slotted or milled into the inner circumferential surface of the feed bush and remain constant in their number, width, depth and length during operation.
The grooves permanently made in the extruder barrel necessitate a relatively high starting torque in start-up of the extruder compared with the conventional system having a smooth extruder barrel. In many cases this requires both a greatly over-dimensioned electric motor with gear, as well as robustly designed components, such as a screw and screw barrel. Both not only drive up costs but, due to the necessarily relatively wide screw flights, are also uneconomical because effective channel surface is lost and the conveying efficiency is reduced.
Associated with the high starting torque is the fact that grooved extruder barrels tend to overheat the material in the feed zone during start-up. This results in a material loss since the stationary operating status is not reached until later and the melt is decomposed more intensely by the elevated temperature. Reaching the stationary status can then take 2 to 3 hours in practice.
In addition to the disadvantages in the start-up process, the rigid, grooved system also has severe disadvantages in stationary operation. The throughput of grooved feed bushes is primarily determined by the frictional relationships on the smooth barrel wall and in the grooves or on the grooved shear surfaces, the feed bush and screw geometries and the screw speed. With adequate feed bush length, the throughput is, on the other hand, independent of the pressure profile in the melt and metering zones. Conversely, this also means, however, that the throughput dictated by the feed zone can no longer be changed by the subsequent zones which are overridden by the feed zone.
For the screw geometry of the metering zone, which is fixed as a result of the material throughput to be expected, a pressure .DELTA.p of the order of magnitude of ##EQU1## is approximately produced. The individual quantities are each specified in the following example. If, for example, the pressure is .DELTA.p.apprxeq.0 bar-as is described in the European Published Application No. A2-0,069,271 and which reflects minimum shear degradation-through-put variations may occur during operation due to altered frictional relationships, for example, due to altered particle structure of the product, processing of several materials in the extruder, different speeds or temperatures or altered bulk density .rho.s in the feed zone. These variations then result in a pressure build-up in the metering zone or, less desirably, a pressure reduction, as is illustrated by the following example: